The present invention relates to exhaust gas purifying systems for internal combustion engines, and more particularly the invention relates to an exhaust gas purifying system in which secondary air is supplied to the exhaust gases flowing into a three-way catalyst containing an oxygen storage material in such a manner that the resulting average exhaust air-fuel ratio is optimum for purification of the noxious components in the exhaust gases.
Generally, there is a close relationship between the noxious component purification percentages of an exhaust gas reactor and the exhaust air-fuel ratio (the weight ratio of the sum of the amount of air drawn into the engine and the amount of secondary air to the amount of fuel supplied). For example, where a three-way catalyst is used which removes the noxious components in exhaust gases by oxidizing CO and HC and reducing NO.sub.x with the same catalyst layer as a medium, its purification characteristic in relation to the exhaust air-fuel ratios becomes as shown in FIG. 1.
Consequently, in order that the three-way catalyst in the form of an exhaust gas reactor of the above type may be operated with high efficiency, the exhaust air-fuel ratio must be maintained in the narrow hatched region shown in FIG. 1.
In view of these circumstances, a system has been proposed in which the oxygen content of the exhaust gases which is closely related to the exhaust air-fuel ratio, is detected by an air-fuel ratio sensor, whereby the amount of secondary air supplied into the intake system or the exhaust system is continuously controlled at a constant speed and the exhaust air-fuel ratio is maintained constant.
However, the control system of this type is disadvantageous in that since the air-fuel ratio must be maintained at a constant value within a narrow range, the construction of the system tends to become complicated and the degree of freedom of the system also tends to be reduced.